Many radio receiving systems continue to operate using analog technology developed over three decades ago. Although performing somewhat satisfactorily for the demands imposed upon it at the time, limitations are apparent, particularly with respect to present day applications. The most apparent of these limitations are evident in a limited dynamic range, imprecision and difficulty in applying new beamforming and direction finding algorithms, electronic noise and jammer vulnerability, etc.
One application where the state-of-the-art technology is noticeably inadequate is in a multielement radio receiving array, more commonly known as a high frequency directional finding system. This system, although acceptable in its time, could greatly benefit from a conversion to digital processing technology. Multielement phased receiver arrays have other special signal processing problems in that the phase relationships between the signals received at each element must be measured and preserved with considerable stability and accuracy in order to permit the beamforming, direction finding, etc. operations to be performed. Often the signals of interest are found to be as much as 120-140 dB weaker than noise and jamming signals. In this context it is not difficult to perceive that a conversion to digital signal processing is desirable to obtain and preserve 20-24 bits of dynamic range in the received signals to assure detection and processing of the signals of interest.
The current state-of-the-art for speed and accuracy in analog-to-digital converters is on the order of eight bits at 75 megasamples per second or six bits at 100 megasamples per second for commercial devices. However, in most analog-to-digital systems the sample and hold function is critical since aperture time and jitter can limit the performance of the entire digitizing system. The speed and dynamic range available for digitizing the radio frequency spectrum in some phased array applications is clearly inadequate, especially when considering that it is generally preferable to digitize prior to sub-banding, mixing, detecting, etc. since the analog processes and associated components are typically incapable of linearity throughout the extremely wide dynamic range spanning 120-140 decibels if necessary.
In addition, for a system that will provide the digitizing of a number of high frequency digital sources to extract their information, the mechanism for having an appropriate speed and dynamic range is needed. Such a system must also address all the phase requirements associated with signal processing for such receiver arrays. Currently the state-of-the-art fails to provide a switching and sampling arrangement having the precisely measured synchronism to accommodate a number of high frequency signal sources to allow the meaningful extraction of the information.
Thus, there is a continuing need in the state-of-the-art for a high speed synchronous sampling of several high frequency sources that is fast and accurate enough to enable the extraction of information without the introduction of errors which would compromise the linearity of the information content.